BEIRUT (Reuters) - Regional Kurdish security forces and pro-Syrian government forces have declared a ceasefire in northeastern Syria, the Kurdish side said, calming a three-day outbreak of violence which killed more than 26 people.

The Kurdish Asayish forces said in a statement that the accord took effect at 3:30 p.m. (1230 GMT) on Friday and a Reuters witness said the truce was holding on Saturday.

The witness and the Syrian Observatory for Human Rights, a Britain-based group tracking the five-year-old war in Syria, said it did not appear that Asayish forces had withdrawn since the start of the truce from any recently-gained territory.

During the fighting, Asayish forces seized control of a number of government-controlled positions in the city of Qamishli, in Hasaka province, as well as its main prison.

A Syrian Kurdish official has said this was the second biggest outbreak of fighting between President Bashar al-Assad's government and regional Kurdish forces since Syria's civil war began in 2011.

Qamishli, near the Turkish border, is mostly controlled by Kurdish security forces, though pro-Assad forces still hold a few areas in the city center, and its airport.

Syrian Kurdish forces now dominate wide areas of northern Syria and set up their own government there. Syria has become a patchwork of areas controlled by the government, an array of rebel groups, Islamic State militants, and Kurdish militia.

Mediators have struggled to get Syria's combatants to honor a Feb. 27 cessation of hostilities deal to enable peace talks to proceed. On Friday, the U.N. special envoy for Syria vowed to take the talks into next week despite a walkout by the main armed opposition with both sides gearing up to escalate the war.

(Reporting by Lisa Barrington in Beirut and Rodi Said in Hasaka, Syria; Editing by Mark Heinrich)